The present invention, comprising both method and apparatus, relates to liquid ink supply systems for ink jet printers.
In ink jet printers of the continuous ink jet type, one or more print heads discharge a stream of uniformly sized and regularily spaced ink drops. These drops are selectively charged in accordance with an input intelligence signal, and thereafter, are either deposited on a print receiving member or caught and collected by a suitable catcher. In most printing applications the ratio of drops used for printing is small compared to total drops discharged from the print head so that much of the ink passing through the print heads is collected by the catcher. This ink is typically recirculated back to the ink storage reservoir. Inasmuch as this recirculated ink has been exposed to air, however, it is very common for air, or gas, to be entrapped in the recirculated ink. In order to obtain clear printing in an ink jet printer, it is necessary that uniform ink droplets be emitted from the print head(s) at a given frequency. When gas or air is contained in the liquid ink supplied to the metering pump, pump stability is adversly affected, ink drop formation uniformity and frequency at the print head is hampered, and print distortion can result.
There has, therefore, been a long-standing need for ink supply systems which stabilize the air and gas in the ink supply to facilitate the proper formation and discharge of uniform and regular ink drops from the print heads.
Examples of attempts to solve the air bubble problem and other ink supply system problems are shown in U.S. Pat. Nos. 4,153,902; 3,761,953; 3,929,071; 4,079,384; 4,067,020; 3,512,173; 2,172,539; 4,170,016; 4,038,667; 3,974,508; and 3,971,038. Other less pertinent, but related, art is shown in U.S. Pat. Nos. 4,187,512; 4,144,537; 4,123,761; 4,011,157; 4,053,901; 3,953,862; 3,708,798; 3,719,952; 3,798,656; 3,805,276; 3,930,258; 4,048,165, 4,121,222; 4,152,710; 4,176,363, and 3,361,150.
One attempted solution to the air bubble problem has been to provide an air trap, or bubble catcher, which comprises a chamber, vented to the surrounding atmosphere, through which the ink flows prior to arriving at the print head. See U.S. Pat. Nos. 3,929,071; 4,153,902; and 4,079,384 for examples of this approach. This air trap solution to the air bubble problem has proved less than satisfactory in that a significant amount of air or gas is not removed from the ink supply, or stabilized, and distortion of the recorded matter has resulted.
Another problem has been the tendency of some printers to become "unprimed," over time, so that auxiliary priming devices have sometimes been employed. See U.S. Pat. Nos. 4,170,016; 4,038,667; 3,974,508; 4,187,512; and 4,123,761.
Other problems have been an inability to maintain a stable fluid pressure at the print head to ensure uniform, clear characters.
There is, therefore, a need for an ink supply system which stabilizes any air or gas entrapped in the ink, has self-priming capabilities, and maintains stable fluid pressure and desired ink jet velocity at the printhead.